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            <h2 class="toc-title">目录</h2>
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        </div><article class="page single"><h1 class="single-title animated flipInX">03 Mybatis 连接池的实现原理</h1><div class="post-meta">
            <div class="post-meta-line"><span class="post-author"><a href="https://wlizhi.cc" title="Author" target="_blank" rel="noopener noreffer author" class="author"><i class="fas fa-user-circle fa-fw"></i>Wlizhi</a></span>&nbsp;<span class="post-category">收录于 <a href="/categories/mybatis/"><i class="far fa-folder fa-fw"></i>mybatis</a></span></div>
            <div class="post-meta-line"><i class="far fa-calendar-alt fa-fw"></i>&nbsp;<time datetime="2020-12-20">2020-12-20</time>&nbsp;<i class="fas fa-pencil-alt fa-fw"></i>&nbsp;约 3370 字&nbsp;
                <i class="far fa-clock fa-fw"></i>&nbsp;预计阅读 7 分钟&nbsp;<span id="/posts/mybatis/03-mybatis-pooled-data-source/" class="leancloud_visitors" data-flag-title="03 Mybatis 连接池的实现原理">
                        <i class="far fa-eye fa-fw"></i>&nbsp;<span class=leancloud-visitors-count></span>&nbsp;次阅读
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        </div><div class="details toc" id="toc-static"  kept="">
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                    <span>目录</span>
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                <div class="details-content toc-content" id="toc-content-static"><nav id="TableOfContents">
  <ul>
    <li><a href="#pooledconnection">PooledConnection</a></li>
    <li><a href="#poolstate">PoolState</a></li>
    <li><a href="#pooleddatasource">PooledDataSource</a>
      <ul>
        <li><a href="#连接的获取-popconnection">连接的获取 popConnection</a></li>
        <li><a href="#连接的归还-pushconnection">连接的归还 pushConnection</a></li>
      </ul>
    </li>
    <li><a href="#小结">小结</a></li>
  </ul>
</nav></div>
            </div><div class="content" id="content"><p>mybatis源码系列文章，示例中带有中文注释的源码 copy 自 <a href="https://gitee.com/wlizhi/mybatis-3" target="_blank" rel="noopener noreffer">https://gitee.com/wlizhi/mybatis-3</a></p>
<p>链接中源码是作者从 github 下载，并以自身理解对核心流程及主要节点做了详细的中文注释。</p>
<hr>
<h2 id="pooledconnection">PooledConnection</h2>
<p>PooledConnection 实现了 InvocationHandler 接口，内部持有了 Connection 对象，是对 Connection 的增强类。</p>
<p>内部封装了连接相关的一些状态信息，以及对 close() 的重新定义。原本 Connection 的 close() 是关闭连接，而这里是将连接归还到连接池。</p>
<p>数据结构如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">class</span> <span class="nc">PooledConnection</span> <span class="kd">implements</span> <span class="n">InvocationHandler</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">String</span> <span class="n">CLOSE</span> <span class="o">=</span> <span class="s">&#34;close&#34;</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">Class</span><span class="o">&lt;?&gt;[]</span> <span class="n">IFACES</span> <span class="o">=</span> <span class="k">new</span> <span class="n">Class</span><span class="o">&lt;?&gt;[]{</span><span class="n">Connection</span><span class="o">.</span><span class="na">class</span><span class="o">};</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="kd">final</span> <span class="kt">int</span> <span class="n">hashCode</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 记录当前连接所在的数据源对象，本次连接是有这个数据源创建的，关闭后也是回到这个数据源
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kd">final</span> <span class="n">PooledDataSource</span> <span class="n">dataSource</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 真正的连接对象
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kd">final</span> <span class="n">Connection</span> <span class="n">realConnection</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 连接的代理对象
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kd">final</span> <span class="n">Connection</span> <span class="n">proxyConnection</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 从数据源取出来连接的时间戳
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kt">long</span> <span class="n">checkoutTimestamp</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 连接创建的的时间戳
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kt">long</span> <span class="n">createdTimestamp</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 连接最后一次使用的时间戳
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kt">long</span> <span class="n">lastUsedTimestamp</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 根据数据库url、用户名、密码生成一个hash值，唯一标识一个连接池
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kt">int</span> <span class="n">connectionTypeCode</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 连接是否有效
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kt">boolean</span> <span class="n">valid</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>增强方法 invoke()：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">class</span> <span class="nc">PooledConnection</span> <span class="kd">implements</span> <span class="n">InvocationHandler</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="nd">@Override</span>
</span></span><span class="line"><span class="cl">	<span class="kd">public</span> <span class="n">Object</span> <span class="nf">invoke</span><span class="o">(</span><span class="n">Object</span> <span class="n">proxy</span><span class="o">,</span> <span class="n">Method</span> <span class="n">method</span><span class="o">,</span> <span class="n">Object</span><span class="o">[]</span> <span class="n">args</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">Throwable</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="n">String</span> <span class="n">methodName</span> <span class="o">=</span> <span class="n">method</span><span class="o">.</span><span class="na">getName</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 针对连接的 close() ，进行替换，连接的关闭，修改为将连接归还到连接池。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="k">if</span> <span class="o">(</span><span class="n">CLOSE</span><span class="o">.</span><span class="na">equals</span><span class="o">(</span><span class="n">methodName</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="n">dataSource</span><span class="o">.</span><span class="na">pushConnection</span><span class="o">(</span><span class="k">this</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="k">return</span> <span class="kc">null</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 对于其他方法，逻辑不变，直接执行目标对象的方法。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="k">if</span> <span class="o">(!</span><span class="n">Object</span><span class="o">.</span><span class="na">class</span><span class="o">.</span><span class="na">equals</span><span class="o">(</span><span class="n">method</span><span class="o">.</span><span class="na">getDeclaringClass</span><span class="o">()))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// issue #579 toString() should never fail
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="c1">// throw an SQLException instead of a Runtime
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="n">checkConnection</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="k">return</span> <span class="n">method</span><span class="o">.</span><span class="na">invoke</span><span class="o">(</span><span class="n">realConnection</span><span class="o">,</span> <span class="n">args</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="n">Throwable</span> <span class="n">t</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">throw</span> <span class="n">ExceptionUtil</span><span class="o">.</span><span class="na">unwrapThrowable</span><span class="o">(</span><span class="n">t</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>在 invoke() 中，如果是 close() 方法，则将连接归还到连接池，而不是关闭连接。以达到连接复用的目的。</p>
<h2 id="poolstate">PoolState</h2>
<p>PoolState 是连接池数据结构的封装，其中有空闲连接列表、活跃连接列表、以及一些统计数据等。</p>
<p>数据结构如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">PoolState</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">protected</span> <span class="n">PooledDataSource</span> <span class="n">dataSource</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="c1">// 空闲的连接池资源集合
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kd">final</span> <span class="n">List</span><span class="o">&lt;</span><span class="n">PooledConnection</span><span class="o">&gt;</span> <span class="n">idleConnections</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ArrayList</span><span class="o">&lt;&gt;();</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 活跃的连接池资源集合
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kd">final</span> <span class="n">List</span><span class="o">&lt;</span><span class="n">PooledConnection</span><span class="o">&gt;</span> <span class="n">activeConnections</span> <span class="o">=</span> <span class="k">new</span> <span class="n">ArrayList</span><span class="o">&lt;&gt;();</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 请求的次数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">requestCount</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 累计的获得连接的时间
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">accumulatedRequestTime</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 累计的使用连接的时间。从连接取出到归还，算一次使用的时间；
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">accumulatedCheckoutTime</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 使用连接超时的次数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">claimedOverdueConnectionCount</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 累计超时时间
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">accumulatedCheckoutTimeOfOverdueConnections</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 累计等待时间
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">accumulatedWaitTime</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 等待次数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">hadToWaitCount</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 无效的连接次数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">long</span> <span class="n">badConnectionCount</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><h2 id="pooleddatasource">PooledDataSource</h2>
<p>PooledDataSource 是 DataSource 的一个池化的简单实现。其中封装了 DataSource 相关的配置信息以及连接池信息 PoolState，
包括最大活跃连接数、最大限制连接数、等等。</p>
<p>数据结构如下：</p>
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<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="cm">/**
</span></span></span><span class="line"><span class="cl"><span class="cm"> * This is a simple, synchronous, thread-safe database connection pool.
</span></span></span><span class="line"><span class="cl"><span class="cm"> * 一个简单，同步的、线程安全的数据库连接池
</span></span></span><span class="line"><span class="cl"><span class="cm"> * @author Clinton Begin
</span></span></span><span class="line"><span class="cl"><span class="cm"> */</span>
</span></span><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">PooledDataSource</span> <span class="kd">implements</span> <span class="n">DataSource</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="kd">static</span> <span class="kd">final</span> <span class="n">Log</span> <span class="n">log</span> <span class="o">=</span> <span class="n">LogFactory</span><span class="o">.</span><span class="na">getLog</span><span class="o">(</span><span class="n">PooledDataSource</span><span class="o">.</span><span class="na">class</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="cm">/** 连接池状态 */</span>
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="kd">final</span> <span class="n">PoolState</span> <span class="n">state</span> <span class="o">=</span> <span class="k">new</span> <span class="n">PoolState</span><span class="o">(</span><span class="k">this</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="kd">final</span> <span class="n">UnpooledDataSource</span> <span class="n">dataSource</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="c1">// OPTIONAL CONFIGURATION FIELDS
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="c1">// 最大活跃连接数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">int</span> <span class="n">poolMaximumActiveConnections</span> <span class="o">=</span> <span class="n">10</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 最大闲置连接数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">int</span> <span class="n">poolMaximumIdleConnections</span> <span class="o">=</span> <span class="n">5</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 最大checkout时长（最长使用时间）
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">int</span> <span class="n">poolMaximumCheckoutTime</span> <span class="o">=</span> <span class="n">20000</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 无法取得连接是最大的等待时间
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">int</span> <span class="n">poolTimeToWait</span> <span class="o">=</span> <span class="n">20000</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 最多允许几次无效连接
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">int</span> <span class="n">poolMaximumLocalBadConnectionTolerance</span> <span class="o">=</span> <span class="n">3</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 测试连接是否有效的sql语句
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="n">String</span> <span class="n">poolPingQuery</span> <span class="o">=</span> <span class="s">&#34;NO PING QUERY SET&#34;</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 是否允许测试连接
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">boolean</span> <span class="n">poolPingEnabled</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="c1">// 配置一段时间，当连接在这段时间内没有被使用，才允许测试连接是否有效
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">protected</span> <span class="kt">int</span> <span class="n">poolPingConnectionsNotUsedFor</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="c1">// 根据数据库url、用户名、密码生成一个hash值，唯一标识一个连接池，由这个连接池生成的连接都会带上这个值
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>	<span class="kd">private</span> <span class="kt">int</span> <span class="n">expectedConnectionTypeCode</span><span class="o">;</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>里面核心方法有两个，分别是 popConnection() 、 pushConnection() 。</p>
<p>popConnection() 用于获取连接，pushConnection() 用于归还连接。</p>
<h3 id="连接的获取-popconnection">连接的获取 popConnection</h3>
<p>连接的获取源码如下：</p>
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<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">PooledDataSource</span> <span class="kd">implements</span> <span class="n">DataSource</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">private</span> <span class="n">PooledConnection</span> <span class="nf">popConnection</span><span class="o">(</span><span class="n">String</span> <span class="n">username</span><span class="o">,</span> <span class="n">String</span> <span class="n">password</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">SQLException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="kt">boolean</span> <span class="n">countedWait</span> <span class="o">=</span> <span class="kc">false</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="n">PooledConnection</span> <span class="n">conn</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">		<span class="kt">long</span> <span class="n">t</span> <span class="o">=</span> <span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">		<span class="kt">int</span> <span class="n">localBadConnectionCount</span> <span class="o">=</span> <span class="n">0</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="k">while</span> <span class="o">(</span><span class="n">conn</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 添加同步控制，锁对象是 PoolState。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="kd">synchronized</span> <span class="o">(</span><span class="n">state</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// 如果还有空闲连接，则直接从空闲连接中获取一个，获取到的这个连接从空闲连接列表中移除。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="k">if</span> <span class="o">(!</span><span class="n">state</span><span class="o">.</span><span class="na">idleConnections</span><span class="o">.</span><span class="na">isEmpty</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// Pool has available connection
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">conn</span> <span class="o">=</span> <span class="n">state</span><span class="o">.</span><span class="na">idleConnections</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">0</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Checked out connection &#34;</span> <span class="o">+</span> <span class="n">conn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34; from pool.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 空闲连接列表中，已经没有空闲连接
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 如果当前活跃连接数，小于最大活跃连接数，直接创建新链接。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="c1">// Pool does not have available connection
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="k">if</span> <span class="o">(</span><span class="n">state</span><span class="o">.</span><span class="na">activeConnections</span><span class="o">.</span><span class="na">size</span><span class="o">()</span> <span class="o">&lt;</span> <span class="n">poolMaximumActiveConnections</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// Can create new connection
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="n">conn</span> <span class="o">=</span> <span class="k">new</span> <span class="n">PooledConnection</span><span class="o">(</span><span class="n">dataSource</span><span class="o">.</span><span class="na">getConnection</span><span class="o">(),</span> <span class="k">this</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Created connection &#34;</span> <span class="o">+</span> <span class="n">conn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34;.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 如果当前活跃链接数，达到了最大活跃连接数，且没有了空闲连接。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 获取最早创建的连接，判断是否超时
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="c1">// Cannot create new connection
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="n">PooledConnection</span> <span class="n">oldestActiveConnection</span> <span class="o">=</span> <span class="n">state</span><span class="o">.</span><span class="na">activeConnections</span><span class="o">.</span><span class="na">get</span><span class="o">(</span><span class="n">0</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="kt">long</span> <span class="n">longestCheckoutTime</span> <span class="o">=</span> <span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">getCheckoutTime</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 如果已经超时
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="k">if</span> <span class="o">(</span><span class="n">longestCheckoutTime</span> <span class="o">&gt;</span> <span class="n">poolMaximumCheckoutTime</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// Can claim overdue connection
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="c1">// 使用连接超时的次数 +1
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="n">state</span><span class="o">.</span><span class="na">claimedOverdueConnectionCount</span><span class="o">++;</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 累计超时时间增加。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="n">state</span><span class="o">.</span><span class="na">accumulatedCheckoutTimeOfOverdueConnections</span> <span class="o">+=</span> <span class="n">longestCheckoutTime</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 累计的使用连接的时间增加。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="n">state</span><span class="o">.</span><span class="na">accumulatedCheckoutTime</span> <span class="o">+=</span> <span class="n">longestCheckoutTime</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 将这个旧的超时连接，从活跃的连接列表中移除。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="n">state</span><span class="o">.</span><span class="na">activeConnections</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">oldestActiveConnection</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 如果这个旧的超时连接，存在尚未完成的事务，进行回滚，如果回滚抛出异常，则忽略异常。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="k">if</span> <span class="o">(!</span><span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">getAutoCommit</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">									<span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">rollback</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">								<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="n">SQLException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">									<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Bad connection. Could not roll back&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">								<span class="o">}</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 没有空闲连接，已达最大活跃连接数，最早的连接已经超时，则在移除超时连接后，创建新的连接。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="n">conn</span> <span class="o">=</span> <span class="k">new</span> <span class="n">PooledConnection</span><span class="o">(</span><span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">(),</span> <span class="k">this</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="n">conn</span><span class="o">.</span><span class="na">setCreatedTimestamp</span><span class="o">(</span><span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">getCreatedTimestamp</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">							<span class="n">conn</span><span class="o">.</span><span class="na">setLastUsedTimestamp</span><span class="o">(</span><span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">getLastUsedTimestamp</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 将旧的超时连接置为无效。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="n">oldestActiveConnection</span><span class="o">.</span><span class="na">invalidate</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">							<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Claimed overdue connection &#34;</span> <span class="o">+</span> <span class="n">conn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34;.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="c1">// 没有空闲连接，已达最大活跃连接数，最早的连接没有超时。只能阻塞。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="c1">// Must wait
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>							<span class="k">try</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="k">if</span> <span class="o">(!</span><span class="n">countedWait</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">									<span class="n">state</span><span class="o">.</span><span class="na">hadToWaitCount</span><span class="o">++;</span>
</span></span><span class="line"><span class="cl">									<span class="n">countedWait</span> <span class="o">=</span> <span class="kc">true</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">								<span class="o">}</span>
</span></span><span class="line"><span class="cl">								<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">									<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Waiting as long as &#34;</span> <span class="o">+</span> <span class="n">poolTimeToWait</span> <span class="o">+</span> <span class="s">&#34; milliseconds for connection.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">								<span class="o">}</span>
</span></span><span class="line"><span class="cl">								<span class="kt">long</span> <span class="n">wt</span> <span class="o">=</span> <span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">								<span class="c1">// 阻塞时间设置为无法取得连接是最大的等待时间。这里的阻塞会释放掉锁，在这期间，有可能会被其他线程唤醒。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>								<span class="n">state</span><span class="o">.</span><span class="na">wait</span><span class="o">(</span><span class="n">poolTimeToWait</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">								<span class="c1">// 累积等待时间累加上以上阻塞的时间。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>								<span class="n">state</span><span class="o">.</span><span class="na">accumulatedWaitTime</span> <span class="o">+=</span> <span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">()</span> <span class="o">-</span> <span class="n">wt</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span> <span class="k">catch</span> <span class="o">(</span><span class="n">InterruptedException</span> <span class="n">e</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="k">break</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// 如果最终连接不为空。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="k">if</span> <span class="o">(</span><span class="n">conn</span> <span class="o">!=</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// ping to server and check the connection is valid or not
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="c1">// 连接有效
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="k">if</span> <span class="o">(</span><span class="n">conn</span><span class="o">.</span><span class="na">isValid</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 真实连接没有自动提交，则回滚掉存在的旧事务。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="k">if</span> <span class="o">(!</span><span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">getAutoCommit</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">rollback</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">						<span class="n">conn</span><span class="o">.</span><span class="na">setConnectionTypeCode</span><span class="o">(</span><span class="n">assembleConnectionTypeCode</span><span class="o">(</span><span class="n">dataSource</span><span class="o">.</span><span class="na">getUrl</span><span class="o">(),</span> <span class="n">username</span><span class="o">,</span> <span class="n">password</span><span class="o">));</span>
</span></span><span class="line"><span class="cl">						<span class="n">conn</span><span class="o">.</span><span class="na">setCheckoutTimestamp</span><span class="o">(</span><span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">						<span class="n">conn</span><span class="o">.</span><span class="na">setLastUsedTimestamp</span><span class="o">(</span><span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 将获取到的连接放入到活跃连接列表中、请求的次数自增、累积获得连接的时间增加。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="n">state</span><span class="o">.</span><span class="na">activeConnections</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">conn</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="n">state</span><span class="o">.</span><span class="na">requestCount</span><span class="o">++;</span>
</span></span><span class="line"><span class="cl">						<span class="n">state</span><span class="o">.</span><span class="na">accumulatedRequestTime</span> <span class="o">+=</span> <span class="n">System</span><span class="o">.</span><span class="na">currentTimeMillis</span><span class="o">()</span> <span class="o">-</span> <span class="n">t</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;A bad connection (&#34;</span> <span class="o">+</span> <span class="n">conn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34;) was returned from the pool, getting another connection.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 连接不为空，但是无效。无效连接统计次数自增。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="n">state</span><span class="o">.</span><span class="na">badConnectionCount</span><span class="o">++;</span>
</span></span><span class="line"><span class="cl">						<span class="n">localBadConnectionCount</span><span class="o">++;</span>
</span></span><span class="line"><span class="cl">						<span class="n">conn</span> <span class="o">=</span> <span class="kc">null</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">						<span class="c1">// 如果获取无效连接的次数 大于 最大闲置连接数+最大重试次数，则抛出异常，否则继续尝试获取连接。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>						<span class="k">if</span> <span class="o">(</span><span class="n">localBadConnectionCount</span> <span class="o">&gt;</span> <span class="o">(</span><span class="n">poolMaximumIdleConnections</span> <span class="o">+</span> <span class="n">poolMaximumLocalBadConnectionTolerance</span><span class="o">))</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">							<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">								<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;PooledDataSource: Could not get a good connection to the database.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">							<span class="o">}</span>
</span></span><span class="line"><span class="cl">							<span class="k">throw</span> <span class="k">new</span> <span class="n">SQLException</span><span class="o">(</span><span class="s">&#34;PooledDataSource: Could not get a good connection to the database.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">						<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="k">if</span> <span class="o">(</span><span class="n">conn</span> <span class="o">==</span> <span class="kc">null</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="k">throw</span> <span class="k">new</span> <span class="n">SQLException</span><span class="o">(</span><span class="s">&#34;PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">		<span class="k">return</span> <span class="n">conn</span><span class="o">;</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>获取的流程：</p>
<ul>
<li>获取添加同步控制，一次只允许一个线程进行连接的获取。这里是线程安全的。</li>
<li>如果空闲连接列表不为空，直接从连接列表头部获取一个连接，并连接从空闲列表中移除。</li>
<li>如果空闲列表为空。
<ul>
<li>如果活跃连接数小于最大活跃连接数，创建一个新的连接。</li>
<li>如果活跃连接数达到最大活跃连接数，获取最早创建的活跃连接，判断是否超时。
<ul>
<li>如果已经超时，设置相关统计数据，如果存在未提交事务，进行回滚。从活跃连接列表中移除，使用旧的真实连接封装一个新的 PooledConnection，
并将旧的 PooledConnection 置为无效。</li>
<li>如果未超时，阻塞线程。</li>
</ul>
</li>
</ul>
</li>
<li>最终如果获取到的连接不为空。
<ul>
<li>如果连接有效，设置连接的一些统计数据，将连接设置到活跃连接列表中。</li>
<li>如果连接无效，判断是否超出最大重试次数，如果超出抛出异常，否则重新尝试获取。</li>
</ul>
</li>
<li>如果获取到的连接为空，打印 debug 日志后抛出异常。</li>
</ul>
<p>流程图：</p>
<p><figure><a class="lightgallery" href="https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png" title="获取连接流程图" data-thumbnail="https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png" data-sub-html="<h2>pop-connection-process.png</h2><p>获取连接流程图</p>">
        <img
            class="lazyload"
            src="/svg/loading.min.svg"
            data-src="https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png"
            data-srcset="https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png, https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png 1.5x, https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png 2x"
            data-sizes="auto"
            alt="https://oss.wlizhi.cc/blog/mybatis/pop-connection-process.png" />
    </a><figcaption class="image-caption">pop-connection-process.png</figcaption>
    </figure></p>
<h3 id="连接的归还-pushconnection">连接的归还 pushConnection</h3>
<p>pushConnection() 源码如下：</p>
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<td class="lntd">
<pre tabindex="0" class="chroma"><code class="language-java" data-lang="java"><span class="line"><span class="cl"><span class="kd">public</span> <span class="kd">class</span> <span class="nc">PooledDataSource</span> <span class="kd">implements</span> <span class="n">DataSource</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl">	<span class="kd">protected</span> <span class="kt">void</span> <span class="nf">pushConnection</span><span class="o">(</span><span class="n">PooledConnection</span> <span class="n">conn</span><span class="o">)</span> <span class="kd">throws</span> <span class="n">SQLException</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">		<span class="c1">// 在归还连接时加上同步控制。锁对象是 PoolState。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>		<span class="kd">synchronized</span> <span class="o">(</span><span class="n">state</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 从活跃连接列表中，移除此连接。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="n">state</span><span class="o">.</span><span class="na">activeConnections</span><span class="o">.</span><span class="na">remove</span><span class="o">(</span><span class="n">conn</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">			<span class="c1">// 如果连接有效
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>			<span class="k">if</span> <span class="o">(</span><span class="n">conn</span><span class="o">.</span><span class="na">isValid</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// 空闲连接数量 小于 最大空闲连接数 且 是从当前连接池中获取到的连接。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="k">if</span> <span class="o">(</span><span class="n">state</span><span class="o">.</span><span class="na">idleConnections</span><span class="o">.</span><span class="na">size</span><span class="o">()</span> <span class="o">&lt;</span> <span class="n">poolMaximumIdleConnections</span> <span class="o">&amp;&amp;</span> <span class="n">conn</span><span class="o">.</span><span class="na">getConnectionTypeCode</span><span class="o">()</span> <span class="o">==</span> <span class="n">expectedConnectionTypeCode</span><span class="o">)</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 累加累积使用连接的时间
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">state</span><span class="o">.</span><span class="na">accumulatedCheckoutTime</span> <span class="o">+=</span> <span class="n">conn</span><span class="o">.</span><span class="na">getCheckoutTime</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 回滚掉可能存在的事务。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="k">if</span> <span class="o">(!</span><span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">getAutoCommit</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">rollback</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 将原始连接封装成一个新的 PooledConnection，
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">PooledConnection</span> <span class="n">newConn</span> <span class="o">=</span> <span class="k">new</span> <span class="n">PooledConnection</span><span class="o">(</span><span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">(),</span> <span class="k">this</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="n">state</span><span class="o">.</span><span class="na">idleConnections</span><span class="o">.</span><span class="na">add</span><span class="o">(</span><span class="n">newConn</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="n">newConn</span><span class="o">.</span><span class="na">setCreatedTimestamp</span><span class="o">(</span><span class="n">conn</span><span class="o">.</span><span class="na">getCreatedTimestamp</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">					<span class="n">newConn</span><span class="o">.</span><span class="na">setLastUsedTimestamp</span><span class="o">(</span><span class="n">conn</span><span class="o">.</span><span class="na">getLastUsedTimestamp</span><span class="o">());</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 旧的 PooledConnection 设置为无效。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">conn</span><span class="o">.</span><span class="na">invalidate</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">					<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Returned connection &#34;</span> <span class="o">+</span> <span class="n">newConn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34; to pool.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 连接归还成功，唤醒其他正在等待获取连接的线程。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">state</span><span class="o">.</span><span class="na">notifyAll</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 空闲连接数量 大于等于 最大空闲连接数
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="n">state</span><span class="o">.</span><span class="na">accumulatedCheckoutTime</span> <span class="o">+=</span> <span class="n">conn</span><span class="o">.</span><span class="na">getCheckoutTime</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">					<span class="c1">// 回滚掉可能存在的事务，将真实连接关闭，连接包装类设置为无效。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>					<span class="k">if</span> <span class="o">(!</span><span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">getAutoCommit</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">rollback</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="n">conn</span><span class="o">.</span><span class="na">getRealConnection</span><span class="o">().</span><span class="na">close</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">					<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">						<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;Closed connection &#34;</span> <span class="o">+</span> <span class="n">conn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34;.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">					<span class="o">}</span>
</span></span><span class="line"><span class="cl">					<span class="n">conn</span><span class="o">.</span><span class="na">invalidate</span><span class="o">();</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span> <span class="k">else</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">				<span class="k">if</span> <span class="o">(</span><span class="n">log</span><span class="o">.</span><span class="na">isDebugEnabled</span><span class="o">())</span> <span class="o">{</span>
</span></span><span class="line"><span class="cl">					<span class="n">log</span><span class="o">.</span><span class="na">debug</span><span class="o">(</span><span class="s">&#34;A bad connection (&#34;</span> <span class="o">+</span> <span class="n">conn</span><span class="o">.</span><span class="na">getRealHashCode</span><span class="o">()</span> <span class="o">+</span> <span class="s">&#34;) attempted to return to the pool, discarding connection.&#34;</span><span class="o">);</span>
</span></span><span class="line"><span class="cl">				<span class="o">}</span>
</span></span><span class="line"><span class="cl">				<span class="c1">// 归还的连接是无效的，则无效连接统计数量 +1。
</span></span></span><span class="line"><span class="cl"><span class="c1"></span>				<span class="n">state</span><span class="o">.</span><span class="na">badConnectionCount</span><span class="o">++;</span>
</span></span><span class="line"><span class="cl">			<span class="o">}</span>
</span></span><span class="line"><span class="cl">		<span class="o">}</span>
</span></span><span class="line"><span class="cl">	<span class="o">}</span>
</span></span><span class="line"><span class="cl"><span class="o">}</span>
</span></span></code></pre></td></tr></table>
</div>
</div><p>代码流程：</p>
<ol>
<li>添加同步控制，只允许一个线程归还连接，其他线程会阻塞。</li>
<li>从活跃连接列表中移除此连接。</li>
<li>如果连接有效。
<ol>
<li>如果空闲连接数小于最大空闲连接数，执行归还操作。归还时会将原始连接封装到新的 PooledConnection 实例中，旧的 PooledConnection 会标记为无效。</li>
<li>如果空闲连接数达到最大空闲连接数，丢弃连接。</li>
</ol>
</li>
<li>如果连接无效，设置统计变量，无效连接统计数量 +1。</li>
</ol>
<p>流程图：</p>
<p><figure><a class="lightgallery" href="https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png" title="归还连接流程图" data-thumbnail="https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png" data-sub-html="<h2>push-connection-process.png</h2><p>归还连接流程图</p>">
        <img
            class="lazyload"
            src="/svg/loading.min.svg"
            data-src="https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png"
            data-srcset="https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png, https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png 1.5x, https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png 2x"
            data-sizes="auto"
            alt="https://oss.wlizhi.cc/blog/mybatis/push-connection-process.png" />
    </a><figcaption class="image-caption">push-connection-process.png</figcaption>
    </figure></p>
<h2 id="小结">小结</h2>
<p>这三个类，实际上就是实现一个简单的连接池,以达到连接复用的目的。一般开发环境不会使用这个，集成到 spring 时一般使用第三方连接池。</p>
<p>连接的获取和归还，同步控制使用的同一把监视锁，他们是互斥的。</p></div>

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